Method for facilitating the identification of icbm nose cones and for discriminating against decoys by spectral analysis



w. B. PLUM 3,327,124 ITATING THE IDENTIFICATION OF ICBM NOSE CONES AND FOR DISCRIMINATING AGAINST DECOYS BY SPECTRAL ANALYSIS June 20, 1967 METHOD FOR FACIL Filed July 3l. 1962 All IPw2mJm wmmDFOPOIa N. Ewan $5.25 mooumQl/@ INVENTOR WILLIAM B. PLUM @wat @L ATTORNEY United States Patent O METHOD FOR FACILITATING THE IDENTIFICA- THON F ICBM NSE CGNES AND FOR DIS- CRIMINATHNG AGAINST DECOYS BY SPECTRAL ANALYSIS William B. Plum, Ventura, Calif., assigner to the United States of America as represented by the Secretary of the Navy Filed July 31, 1962, Ser. No. 214,466 2 Claims. (Cl. Z50-226) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to a method and means for facilitating the correct identification of objects passing at a relatively high speed through the earths atmosphere.

The detection of an `Intercontinental Ballistic Missile (ICBM) during the early part of its re-entry phase is of extreme importance at the present time since any eifective defense is obviously dependent thereupon. The problem is rendered more difficult of solution by the probable presence of other objects which it must be assumed will be employed as decoys in the event of an enemy attack. Even if such decoys are not utilized, it is possible that the tracking radar will pick up (or lock on to) the ICBM tankage fragments rather than on to the nose cone which carries the warhead. Furthermore, infra-red detectors may yield false information, since a small object can be designed to burn at a high temperature, thereby simulating the thermal radiant power of a nose cone. It is with this general situation that the present invention is concerned.

Experiments with a large number of materials under widely varying conditions have demonstrated that the most satisfactory substance for the fabrication of ICBM nose cones is of an organic nature, such, for example, as that class represented by the phenolics. These organic substances are constituted largely lof carbon, hydrogen and oxygen atoms. Upon entering the earths atmosphere on a descending trajectory, the organic nose cone becomes heated due to air friction, and the rise in the temperature experienced by an object in s-uch an environment is known to exceed l200 K. This temperature is suiciently high to result in the liberation of hydrogen gas from the organic material of which the nose cone is constituted. The extreme temperature developed by the passage of the nose cone through the atmosphere is also above the autoignition point for a mixture of hydrogen and oxygen, and consequently a hot ame is produced. All of the conditions are thus present to bring about an ionization of the hydrogen atom.

It is well known to workers in the field to which the present invention relates that an ionized hydrogen atom, upon capturing an electron, normally radiates two strong spectral lines in the Balmer series, namely, at wavelengths of 4861 and 6562 Angstrom units. The former is dicult to distinguish from the black-body radiation emitted by the heated gas, but the 6562 A. spectral line is not so masked. The inclusion of this characteristic in the atomic radiation received from an organic nose cone or other object passing through the earths atmosphere may consequently be employed to distinguish such an object from other masses composed of materials having spectra in which this characteristic is not present.

One object of the present invention, therefore, is to provide a method and means for facilitating the identitication of an object passing at a relatively high speed through the earths atmosphere.

Another object of the invention is to provide a method and means for facilitating the identification of an Intercontinental Ballistic Missile (ICBM) nose cone from decoys or tankage fragments in cases where the nose cone is fabricated from some known substance such as an organic material.

A further object of the invention is to provide for the spectral analysis of atomic Aor spectral radiation emitted by a body passing through the earths atmosphere.

A still further object of the invention is to provide means for receiving atomic Ior spectral radiation emitted by a body passing at high speed through the earths atmosphere, this receiving means acting to yield an `output indication whenever the energy so received contains an abnormally high percentage of radiation occurring at a particular wavelength.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a wavelength vs. in-tensity graph useful in explaining the principles of the present invention;

FIG. 2 is a schematic illustration of a preferred objectidentitication system incorporating principles upon which the present concept is based;

FIG. 3 is a schematic diagram of a preferred form of electrical discrimination network which may form part of the system of FIG. 2.

When organic materials are subjected to high-intensity thermal radiation, high speed thermal jets are produced and free hydrogen gas is liberated. The amount of gas generated in this manner is dependent upon the absolute temperature of the material. Purely as an example, an organic mass subjected to a radiant power of watts/ cm.2 liberates live times as much hydrogen gas as a radiant power exposure of 8 watts/cm.2. It is known that a radiant power of 80 watts/ cm.2 is given olf by a blackbody at a temperature of l950 K., while a radiant power of 8 watts/cm.2 is emitted by a black-body at 1100 K. It is also known in the art to which this invention relates that the autoignition temperature of a mixure of hydrogen and oxygen is 850 K. Thus it follows that any thermal radiant power above approximately 3 watts/ cm2 will ignite a hydrogen-oxygen mixture.

The present state of missile technology recognizes many advantages inherent in the fabrication of heat shields for re-entry bodies fabricated of `organic material, these heat shields being commonly of the ablative type. In passing through the earths atmosphere at high speeds, these objects become heated to a temperature above 1200" K. due to air friction, and hydrogen gas is liberated by pyrolysis. Since this temperature of 1200 K. is above the autoignition point for a mixture of hydrogen and oxygen, a hot flame is produced, atomic radiation occurs, and such radiation can be received and spectrally analyzed.

In FIG. l of the drawings is shown a graph of intensity distribution as a function of wavelength for a re-entry body (such, for example, as an ICBM nose cone) fabricated of organic material. It will be seen that the basic radiation has a continuous spectrum as represented by the curve 10, this curve being illustrated for a particular arbitrary temperature higher than 850 K. As will be noted, the spectral intensity curve 10 rises rather sharply to a maximum and then tapers off more gradually at longer wavelengths. While the location of the maximum point depends upon the temperature of the radiator, the exact shape of the curve is unimportant from the standpoint of the present disclosure.

It has been mentioned above that an organic material will radiate a number of spectral lines in the Balmer series, two strong lines occurring at wavelengths of 4861 and 6562 Angstrom units, the latter spectral line being representative of Hot (red). This particular spectral line is not present in the radiation emitted from masses from which no hydrogen ionization takes place.

The above condition is utilized in accordance with the present invention as the basic feature in a system for discriminating between various types of re-entry bodies. For example, referring to FIG. 2 of the drawings, there is shown a device generally identified by the reference numeral 12 which is adapted to receive atomic radiation emitted by some re-entry object such as an Intercontinental Ballistic Missile nose cone 14. The device 12 is arranged in a known fashion for universal movement so that it may be optically aligned with any point within its operating range. An apparatus of this nature is commonly used for missile tracking purposes, and hence the structural details will not be set forth therein as they form no part of the present concept. It is only necessary to appreciate that the device 12 may be oriented so as to be directed at any atmospheric object of interest. Obviously the device 12 to serve a useful purpose must have an extremely limited field of view in any one position, preferably not exceeding a figure in the order of one minute of arc, and one means for achieving this objective will be set forth at greater length in connection with a description of the apparatus of FIG. 3.

The principal objective of the present disclosure is to provide a method and means for discriminating between an actual Intercontinental Ballistic Missile nose cone, such as represented in FIG. 2 by the reference numeral 14, and a decoy (or tankage fragments) such as represented by the reference numeral 16. It is assumed that in either case the re-entry object 14 and/or 16 will be traveling at a sufficiently high rate of speed so that pyrolysis will occur and atomic or spectral radiation will be emitted therefrom. The function of the device 12 is to receive this atomic or spectral radiation and to make use thereof to yield an indication as to the true identity of the object from which the atomic radiation is emitted.

In carrying out this objective, the device 12 incorporates a pair of optical filters F1 and F2, and radiation from any particular object being investigated is intended to pass through both of these filters. As best shown in FIG. 1, filter number #l (F1) is designated to pass a very narrow band of radiation centered about the 6562 Angstrom (or Ha) spectral line (iapproximately Angstrom units) this band including a portion of the black-body radiation having a continuous spectra the varying amplitude of which is represented by the curve 1f). Filter number #2 (F2) is likewise designed to pass a narrow (i5 Angstrom units) band of radiation at about 6500 A. Filter #1 should have as narrow a pass band as is practicable, for reasons which will later become apparent. The actual wave band covered by the lter F2 is not critical, and its limits may be varied quite widely. Filter F1, however, is designed to pass substantially only a wavelength of 6562 Angstroms.

Referring now to FIG. 3, the filters are illustrated as respectively passing therethrough atomic radiation emitted by the re-entry bodies 14 and 16 of FIG. 2, the filters each incorporating some known lmeans such as a grading 17 for limiting the field of view, in an optical sense, to a very small angle, preferably not exceeding one minute of arc. Assuming for the purpose of explanation that the object being investigated is not formed of organic material, then the radiation therefrom may be such as represented by the curve of FIG. 1, or, in other words, it will be that of a black-body. Upon the reception of such black-body radiation, it will pass through both filters, the energy emerging from filter #l being received by a phototube 18, and the energy being passed by filter #2 being similarly received by a second phototube 20. As will be recognized from an inspection of FIG. 3, phototube 1S (which may be of the conventional photomultiplier type having a photocathode) forms part of a circuit which includes a source of energy 22 and the resistance element 24 of a potentiometer, these components 18, 22 and 24 being series-connected in the manner shown so that when phototube 18 is conductive energy will flow through the resisitance element 24. In similar fashion, phototube 20 forms part of a series circuit which includes a source of energy 26 and the resistor 28.

The respective photocathodes of tubes 18 and 20 are interconnected through a resistor 30, and the series combination of a resistor 32 and an indicating device 34 (such as a null meter) is connected between the negative terminal of energy source 26 and the movable element 36 of the potentiometer which includes resistance element 24, It will now be recognized that the components 24, 28, 30, 32 and 34 form a bridge circuit. When radiation from a blackbody is received by the device 12, different amounts of energy will pass through the respective filters #l and #2. While this steady condition prevails, the movable potentiometer element 36 is adjusted so that the indicating device 34 reads zero. If now the device 12 is loriented to receive atomic or spectral radiation from a re-entrant body composed of organic material, a strong spectral line at 6562 A. will be included in the received radiation. Energy of this particular wavelength will cause a marked increase in the conduction of phototube 18 and little if any increase in the conduction of phototube 20. The bridge circuit included in the discrimination network will thus become unbalanced, and the indicator 34 will depart from a zero reading. This will represent to an observer that the re-entrant body being investigated possesses certain constitutents which may aid in the identifications thereof, and this in turn may affect the decision as to what, if any, action should be Vtaken in a countermeasures sense.

Although not specifically set forth in the drawing and in the preceding description, it is contemplated that the means of present disclosure be employed in conjunction with a tracking radar to provide the radar operator with positive assurance that the object being tracked is an ICBM nose cone of the ablative type rather than a decoy or tankage fragments. Under such circumstances, the device 12 of FIG. 2 may advantageously be mounted for movement concurrently with the radar scanning mechanism, the optical axis of the device 12 being aligned parallel to the search axis of the radar.

Instead of a single filter F2, passing a narrow band of energy lower in wavelength than that passed by filter F1, a pair of filters may be employed equally spaced on either side of F1 and F2 (as shown in the drawing) due to the falling slope of curve 10. However, in most cases a proper adjustment of potentiometer element 36 can take care of any such amplitude difference which may exist.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

I claim:

1. The method of discriminating between two objects each of which is passing through the earths atmosphere at a speed sufificiently high to produce pyrolysis, said method including the steps of spectrally analyzing the radiation emitted by ench object, and then deriving an indication whenever the radiation emitted by a particular one of the objects possesses a predetermined spectral characteristic not present in the spectrum of the radiation emitted by the other of said objects.

2. The ymethod of identifying an ICBM nose cone composed of organic material when the latter is accompanied by a decoy during re-entry into the earths atmosphere, said nose cone undergoing pyrolysis during such re-entry phase to result in a liberation of hydrogen gas and consequent spectral radiation caused by autoignition of such hydrogen gas with oxygen of the atmosphere, said method including the steps of receiving spectral radiation from said nose cone as well as atomic radiation from said decoy and then respectively analyzing the radiation so received to determine the presence or absence of a hydrogen line 5 in the spectrum of eacn individual object, such hydrogen line, when present, being indicative of the organic composition of the object from which that particular spectral radiation is emitted.

References Cited UNITED STATES PATENTS 9/ 1949 Neufeld Z50-226 6 Jacobs et a1. Z50-210 Ator Z50-210 Thomsen Z50-210 Pelavin Z50-226 X RALPH G. NILSON, Primary Examiner.

SAMUEL FEINBERG, Examiner. T. A. ROBINSON, J. D. WALL, Assistant Examiners. 

1. THE METHOD OF DISCRIMINATING BETWEEN TWO OBJECTS EACH OF WHICH IS PASSING THROUGH THE EARTH''S ATMOSPHERE AT A SPEED SUFFICIENTLY HIGH TO PRODUCE PYROLYSIS, SAID METHOD INCLUDING THE STEPS OF SPECTRALLY ANALYZING THE RADIATION EMITTED BY ENCH OBJECT, AND THEN DERIVING AN INDICATION WHENEVER THE RADIATION EMITTED BY A PARTICULAR ONE OF THE OBJECTS POSSESSES A PREDETERMINED SPECTRAL CHARACTERISTIC NOT PRESENT IN THE SPECTRUM OF THE RADIATION EMITTED BY THE OTHER OF SAID OBJECTS. 